Is there a problem with Plusnet to Plusnet calls right now?

[#1]

I am on Plusnet home phone and so is my mother.
If I call her Plusnet phone from any NON Plusnet landline, it works fine. However, if I call her on my Plusnet landline (which connects to other NON Plusnet phone without problem) I get an unobtainable (fast beeps - as opposed to engaged) tone.
My problem is therefore only with Plusnet <-> landline calls.
Anybody else?

[#2]

Just had a call from my daughter....no problems (both on PN home phone).

[#3]

'Number Unobtainable' (NU in the trade) is a continuous tone. The only 'fast beeps' that come to mind is pay tone, as from a payphone - but that seems highly unlikely. 

[#4]

Calls are routed for PN > PN customers just the same as any BTOR > BTOR customer i.e. BT > BT, BT > PN, Post Office > PN, etc. etc. As such, there's no reason for PN > PN to be broken. If you're having issues dialling a number, it's best to speak to Phone Faults via the usual support mechanisms.