A common problem in the UK is that companies Active Directory telephone numbers will be populated in the format:
+44 (0) 123 456 7891 or +44(0)1234567891
If you were to dial these numbers in the UK you would omit the +44 and just dial the 0. If you are abroad you would dial +44 and omit the 0 (E164 format).
As part of the default Lync normalization rules it will remove all spaces and brackets (). When using Lync click to call from a users contact card this normalizes the number to +4401234567891 which is invalid. Therefore you need to either correct all numbers in AD to E164 format or add a normalization rule to the Company_Phone_Number_Normalization_Rules.txt file on your Lync File Share. This is already well documented here:
I did find some other posts about stripping the brackets and zero but I found these rules didn’t account for if there was a space before or after the brackets and anywhere else in the number. This is the normalization rule I use:
# Normalize UK numbers that have a +44 (0) prefix with or without spaces
# +44 (0) 123 456 7891 would become +441234567891
# +44(0)1234567891 would become +441234567891