The best bet is to try separation of the phone wires if they are near the antenna/feedline. Next would be curative measures at the phone end. The ham radio magazine "QST" has ads for telephone filters and phones that are bullet proofed. The magazine can probably be obtained at your local library or just do an internet search. I don't know if the filters are designed for commercial type phones or just residential. In either case, most of the folks selling these devices used to work in the telco/communications industry and might be able to give you some detailed information specific to your problem.
It's a sad thing that there are no mandatory RF immunity standards in this country; we are the only industrialized country in the world without any. I used to hear this type of complaint just about everyday and the complainants don't want to hear they have to cure the problem. I worked for the FCC for 30+ years in the Saint Paul MN field office, so I am quite familiar with the issues.
