# BT-57 Buyer contact telephone number

Telephone number for the buyer contact point.

## Quick answer

Telephone number for the buyer contact point.

> **Buyer details**: Who receives the invoice and which buyer records or routing data must match.

## In plain language

Telephone number for the buyer contact point.

Telephone number for the buyer contact point.

Provide this only when the invoice recipient expects telephone contact details in the structured invoice.

## What you usually enter here

Telephone number

Typical valid values include: +49 30 123456

Optional according to EN 16931-1.

## What goes wrong often

Avoid values like:

## Examples in context

Provide this only when the invoice recipient expects telephone contact details in the structured invoice.

Optional according to EN 16931-1.

## Valid Examples

- +49 30 123456

## Related Fields

Provide this only when the invoice recipient expects telephone contact details in the structured invoice.

- [BT-56 Buyer contact point](/resources/xrechnung/bt-56-buyer-contact-point)
- [BT-58 Buyer contact email address](/resources/xrechnung/bt-58-buyer-contact-email-address)

## XML mapping and field facts

Open the technical reference if you need XML paths, validator rule codes, legal wording, or schema-specific mapping details.

### Field ID:
BT-57

### Data Type:
text

### Cardinality:
0..1

### Requirement:
Optional

### Max Length:
50 characters

### Since Version:
Optional

## XML Implementation

### UBL 2.1 Syntax
```
/Invoice/cac:AccountingCustomerParty/cac:Party/cac:Contact/cbc:Telephone
```

### CII (UN/CEFACT) Syntax
```
/CrossIndustryInvoice/SupplyChainTradeTransaction/ApplicableHeaderTradeAgreement/ram:BuyerTradeParty/ram:DefinedTradeContact/ram:TelephoneUniversalCommunication/ram:CompleteNumber
```

## Why the standard cares about this field

### Business Context
Provide this only when the invoice recipient expects telephone contact details in the structured invoice.

### Legal Requirement
Optional according to EN 16931-1.

## Official sources and tools

- [KoSIT Validator](https://erechnungsvalidator.service-bw.de/)
- [Invoice Converter](/pdf-to-xrechnung)
- [XRechnung Standard](https://xeinkauf.de/xrechnung/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)