Is there an issue with telephone lines by other than the regular analog telephone provider when used for fire alarm monitoring (DACT) connections? For example lines provided by Verizon or Time Warner?

Primary line must be a wired land line and can't be shared or seized by anything else. Secondary can be VOIP, and can be shared. Provider doesnt matter as long as there is a number that isn't shared or seized by anything else.

The question was based on the concern that Time warner may convert the signel to digital (not my concern).

But since you brought it up - I was not aware that the primary line could not be shared by anything else.
Not even a fax machine?
Security system?
Elevator call out?

Since few have the alarm code, perhaps we might post the section, let us see the words?

6. Can non‐traditional telephone service be used with a digital alarm communicator
system?
Yes, if the service is provided through a managed facilities‐based voice network (MFVN).
NFPA 72 addresses several means of transmitting alarm and other signals from a fire
alarm system at a protected premise to a supervising station. The most widely used
means is a digital alarm communicator system. These systems use a digital alarm
communicator transmitter (DACT) connected through two transmission channels. The
Code requires the primary channel to be a telephone line. The secondary channel can
be a second telephone line, or one of 6 other transmission means.
With respect to the telephone line(s), the Code requires in 26.6.3.2.1.1 that the DACT be
connected to the public switched telephone network upstream of any private telephone
system at the protected premise. It also requires that the connection be to a loop start
telephone circuit. Historically, this meant connection to the copper conductors of a
plain old telephone system (POTS) traditionally provided by the telephone company. In
recent years, providers of telephone service other than the traditional POTS service
have become more common. The 2010 edition of the Code includes revisions to
address the use of these non‐traditional types of telephone service.
The 2010 edition of NFPA 72 includes the following revised the definition of a public
switched telephone network and a new definition of managed facilities‐based voice
networks:
 Public Switched Telephone Network (PSTN). An assembly of communications
equipment and telephone service providers that utilize managed facilities‐based
voice networks (MFVN) to provide the general public with the ability to establish
communications channels via discrete dialing codes.
 Managed Facilities‐Based Voice Network (MFVN). A physical facilities‐based
network capable of transmitting real time signals with formats unchanged that is
managed, operated, and maintained by the service provider to ensure service
quality and reliability from the subscriber location to public switched telephone
network (PSTN) interconnection points or other MFVN peer networks.
The Code contains extensive explanatory annex information related to what is expected
of a MFVN. It is important to recognize that telephone service that that is not provided
through a physical facilities‐based network would not be covered under this definition.
It is also important to understand that the telephone service provider’s communications
equipment is expected to provide 8 hours of standby power for equipment installed on
the premises or located in the field. This is in contrast to the 24 hours of secondary
power required for the fire alarm system itself, including the DACT.
Those using or enforcing earlier editions of the Code will find the revisions in the 2010
edition of NFPA 72 useful in understanding the capabilities and performance expected
for the communication pathways used in conjunction with digital alarm communicator
systems.