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Are you able to check that you are wnt accidentally blocked them in your text messaging app @Shellwestwood75
Also are you sending long texts which could be then converted to MMS or are they short texts ie 160 characters with no pictures attached etc
Current Phone > Samsung Note 10+ 5G _256Gb > Model: SM-N976B.
Samsung One Ui 2.1 / Android 10.
Samsung Gear s3 Frontier Watch.
Samsung Galaxy Buds.
If the outgoing message to one person is marked as delivered, this will be a problem at the receiving end and the person you are sending this message to needs to check with the receiving network for the problem.
Your message is leaving the Vodafone network and delivered to the receiving network message centre for forwarding onto the recipient but delivery from the receiving message centre is failing to the phone.