Calls cutting off after 30mins

[#1]

Just called my parents tonight, both times the call cut off after 30mins 10s ... of course I have no idea if this is a PN (my phone) or TT (thier phone) issue.  For anybody looking at my calls logs, the number dialed which was cut off ends in 14.
SW.

[#2]

Bizarre - Never heard of that happening on our service before.
Do either you or your parents makes other calls of that length?  Might be an idea finding out if it's just between you two or if it happens on other calls aswell.