XP dial up issues

I work at a pc comapny and we have had a lot of problems with dynalink modems and not connecting very well etc so we had to slow them down,that worked ok, then we replaced with a different brand which works ok, this is not the problem tho, I have had a couple of systems now where the modenm will not dial out at all, even after replacing and adjusting different settings, this happened only after they had been in the systems for a couple of months, tried an external modem on one and dials fine every time.
Any ideas would be greatly appreciated.
Especially as they worked fine for several months.
Thanks