Odd problem with Neuf

[Pommier]

We have Neuf for broadband including free phonecalls, and it's been working fine.

However, since Tuesday when I phone my mother (in France) I always get the engaged tone, even though most times her phone hasn't been engaged. I can call any other number as normal and there's no problem receiving calls from her or anyone else. Her phone is also receiving and making calls as usual and FT have tested her line and say it's working fine.

Anyone got any ideas?

Cheryl