Regular Irish landlines (and NI landlines using the 048 prefix) and Irish mobiles can only be called using membership or add-on bundled minutes and cannot be charged from your credit balance. The cost of add-ons are detailed here:
http://www.48months.ie/boiler-plate/add-on-rules
Charges for calls not covered by membership/add-on bundled minutes are detailed here:
Other call charges
If you think that your bundled minutes have run out too quickly then log a support query to ask 48 support for details of your call usage and it should clarify.
How to get help
Hope this helps.
