Panasonic KX-TW501 busy tone? (receives OK)

Hi,

I have a Panasonic KX-TW501 (GSM Fixed Wireless Phone): https://www.vodafone.co.nz/MEDIA_CustomProductCatalog/m2620179_Panasonic-TW501-Userguide.pdf

I bought a new prepay SIM card to use. It receives calls and text messages no problem.

When I try and make a call, I get a busy tone or no response at all. If I try and send a text message it tries and fails without an error.

Any ideas?

Hope was to give it to my granddaughter to use as a home phone until she is old enough to have a mobile phone.

Regards,

Andrew